Preparation of polyisocyanates from polycarbamates

ABSTRACT

In the process of producing polyisocyanates by 
     (a) condensing an alkyl-N-phenylcarbamate having 1 to 3 carbons in the alkyl moiety in the presence of an acid to produce condensate containing diphenylmethane dicarbamates and polymethylene polyphenyl carbamates with by-product N-benzyl compounds, rearranging said N-benzyl compounds in said condensate with acid catalyst to obtain a pyrolysis feed mixture containing diphenylmethane dicarbamates, polymethylene polyphenyl carbamates and 0.02 to 1.0 percent by weight of amine impurities as NH 2  groups, and 
     (b) thermally decomposing the carbamate moieties in the pyrolysis feed mixture to isocyanate moieties to produce polyisocyanates, 
     the improvement comprises increasing the percent isocyanate content of said polyisocyanates by prior to step (b) converting any amine salt by-products in said pyrolysis feed mixture to free amine by-products by contacting the mixture with a weakly basic tertiary amine anion exchange resin and removing any free amine by-products by contacting the feed mixture with a strongly acidic sulfonated polyaromatic ion exchange resin whereby the amine impurity in the pyrolysis feed mixture is reduced to less than 0.001 percent by weight as NH 2  groups.

FIELD OF THE INVENTION

The present invention relates to an improvement in the process for thepreparation of polyisocyanates from polycarbamates (polyurethanes). Theimprovement relates to removal of amine and amine salts from thepolyurethane prior to its pyrolytic decomposition to polyisocyanatewhich results in higher levels of isocyanate content.

BACKGROUND OF THE INVENTION

Polymeric aromatic carbamic acid esters (polyurethanes) such asdiphenylmethane dicarbamates and the related higher homologs,polymethylene polyphenyl carbamates, have become increasingly importantproducts particularly, for use in the preparation of the commerciallyvaluable diphenylmethane diisocyanates and mixtures of diisocyanates andthe polyisocyanates by the decomposition of such polymeric aromaticcarbamic acid esters in a suitable solvent as shown in Rosenthal et al.,U.S. Pat. Nos. 3,962,302 and 3,919,279.

A proposed prior art process for the preparation of polymeric aromaticcarbamic acid esters (polyurethanes) is disclosed in Klauke et al, U.S.Pat. No. 2,946,768 and involves the condensation of aryl carbamic acidesters with carbonyl compounds in a dilute aqueous acid condensationmedium. However, in such process the carbonyl compound such asformaldehyde tends to react at the nitrogen of the carbamate to producealong with desired polyurethanes, varying amounts, i.e., generallybetween 15 percent and 50 percent by weight, of undesirable(alkoxycarbonyl)phenylaminomethylphenyl compounds which includes thevarious dimers, trimers, tetramers, etc. of such compounds (alsoreferred to herein as "N-benzyl" compounds). Attempts to prepare mono ordiisocyanates and polyisocyanates or to otherwise use the mixturecontaining the undesired N-benzyl compounds, which cannot be convertedto an isocyanate by pyrolysis, and polyurethanes presents many problems.However, the undesired N-benzyl compounds may be catalyticallyrearranged to a desired polyurethane in accordance with the teachings ofShawl et al, U.S. Pat. No. 4,146,727. Accordingly, a product mixturefrom a condensation as disclosed in aforementioned U.S. Pat. No.2,946,768 containing diurethanes and polyurethanes, N-benzyl compounds,unreacted alkylphenylcarbamates and other by-products such as amines maybe contacted at temperatures of from about 50° C. to 170° C. with aprotonic acid medium having a strength at least equal to a 75 percentsulfuric acid such as concentrated sulfuric acid or an acid mediumcomprising a Lewis acid having a concentration of at least 0.5 percentby weight based on the total reaction mixture, while maintaining aminimum amount of water in the system, to catalytically convert orrearrange said N-benzyl compounds.

Shawl, U.S. Pat. No. 4,172,948, discloses a similar rearrangement ofN-benzyl compounds may be achieved by use of anhydrous hydrogen chlorideunder super atmospheric pressure.

Condensation of aryl carbamic acid esters with formaldehyde may also beconducted with organic sulfonic acids. Shawl, U.S. Pat. No. 4,162,362,teaches that condensation in the presence of an organic sulfonic acideliminates formation of N-benzyl compounds and suppresses certain otherundesirable side reactions.

SUMMARY OF THE INVENTION

The acid catalyzed condensation of N-aryl carbamates and the acidrearrangement of N-benzyl compounds produce some hydrolysis of urethanegroups to amino groups. Thus, for example, in the acid catalyzedcondensation of ethyl-N-phenyl carbamate with formaldehyde somehydrolysis of the urethane (carbamate) groups occurs and the aminocompound would thus correspond to the carbamate from which it is derivedand may amount to 0.02 to 1.0 percent by weight of amine impurity as NH₂groups. Although hydrolysis of the starting ethyl-N-phenyl carbamate toaniline can occur, the most troublesome amines have methylene bridgedphenyl moieties with each phenyl having a carbamate or aminosubstituent.

By-product amino compounds may be present in thecondensation-rearrangement product as free amines and as amine/acidsalts. It has been discovered that when the by-product amines and aminesalts accompany the condensation-rearrangement product to its pyrolyticdecomposition to polyisocyanate, a significant detrimental effectresults. For example, the amines and amine salts can react withisocyanate groups as they form to produce ureas or biurets.Additionally, such undesirable ureas might at elevated temperaturescatalyze isocyanate reactions to produce other unwanted by-products suchas carbodiimides and isocyanurates.

It has now been discovered that by amine removal prior to pyrolysisthose detrimental effects may be avoided. Amine removal is achieved bycontacting the condensation-rearrangement product first with a weaklybasic tertiary amine ion exchange resin and then with a strongly acidicsulfonated polyaromatic ion exchange resin. Alternatively, the productmay be contacted by a bed of a mixture of both types of ion exchangeresins. That treatment removes both amines and amine salts from thecondensation-rearrangement product.

Accordingly it is an object of this invention to provide a process withincreased yield of polyisocyanates from polyurethanes by removing aminesand amine salts from the polyurethane prior to pyrolytic decomposition.

It is another object of this invention to provide a process forproducing polyisocyanates with an increased percentage of isocyanategroup content.

These and other objects and advantages of this invention will becomeapparent from the description of the invention which follows and fromthe claims.

DESCRIPTION OF THE INVENTION

Condensation of alkyl-N-aryl carbamates with formaldehyde is known toyield polyfunctional carbamates with alternating methylene moieties andN-aryl carbamate moieties as shown by the formula ##STR1## wherein x, yand z when the same are each hydrogen or when different x, y and z maybe hydrogen, alkyl having 1-3 carbon atoms, --NHCOOR, --CH₂ ArNHCOOR, or--N(COOR)CH₂ Ar;

n is at least one;

R is alkyl having 1-3 carbon atoms and

Ar is phenyl which is unsubstituted or substituted with alkyl having 1-3carbon atoms.

Production of polyfunctional carbamates by acidic condensation of amonofunctional N-aryl carbamate with formaldehyde results in a smallamount of the carbamate functional groups being hydrolyzed to aminogroups as shown by the reaction ##STR2## wherein Ar and R are as definedabove, the polycarbamate is of formula (I) above and R' is an organicmoiety containing methylene bridged aromatic rings bearing carbamate orother amino substituents. In order for the process of pyrolyzingdicarbamates and polymethylene polyphenyl carbamates (urethanes) to be aviable process it is imperative, because of the high temperaturesrequired for such pyrolysis and the inherent instability of isocyanategroups produced at high temperatures, that the urethanes be of thehighest possible purity and thus provide yields of isocyanate (NCO)groups of at least 95 percent. The aromatic amine by-products which areformed by hydrolysis of the urethane groups during acid condensation andrearrangement reactions are a particularly detrimental class ofimpurities. As much as 1 to 2 percent of the urethane groups may behydrolyzed, resulting in from about 0.02 to 1.0 percent by weight ofamine impurities as NH₂ groups in the reaction product. In urethanepyrolysis the amines can react with the formed isocyanates to give ureaswhich may also form biurets. The amines, or products derived therefrom,can act catalytically to form other undesirable by-products such ascarbodiimides and isocyanurates.

As shown by reaction 1 above, the amine by-product may be present as thefree amine or as the amine salt of an acid catalyst from thecondensation-rearrangement process for producing the polycarbamate. Suchacids, represented by HX in reaction 1. above, are known in the art andinclude strong mineral acids such as sulfuric acid.

Both the free amine and the amine salt can be effectively removed fromthe polycarbamate condensation-rearrangement reaction product bycontacting the reaction product in solution with a mixed bed (monobed)of a weakly basic tertiary amine ion exchange resin and a stronglyacidic sulfonated polyaromatic ion exchange resin. That treatmentresults in a urethane solution with less than 0.001 weight percent ofamine impurity as NH₂ groups. The sulfonic acid acidic ion exchangeresin alone would be effective to remove the free amine but would noteffectively remove amine salts. Most of the amine by-product would bepresent in the salt form because the condensation-rearrangement reactionis acid catalyzed. Thus both types of resins are required for completeamine by-product removal.

By using a mixed bed of a basic tertiary amine ion exchange resintogether with an acidic sulfonated polyaromatic ion exchange resin, thefollowing reactions occur with the undesirable amine by-products:##STR3## The basic ion exchange resin converts the by-product amine saltto free by-product amine and concurrently removes acid HX by saltformation with the basic ion exchange resin as shown in reaction 2. Thefree amine by-product resulting from reaction 2. as well as any otherfree amine reacts with the acidic sulfonated polyaromatic ion exchangeresin as shown in reaction 3. Because the amine is ionically bonded tothe resin, it is effectively removed from the condensation-rearrangementreaction product.

Although use of both acid and basic ion exchange resins mixed in asingle bed (monobed) is preferred, a two step sequential treatment withan individual bed of each resin may be employed. When using individualbeds, it is imperative to contact the condensation-rearrangementreaction product first with the basic ion exchange resin to effectreaction 2. prior to contact with the acid ion exchange resin (reaction3.). It is readily apparent that reversal of the order would defeat thecomplete removal of amine by-product.

The ion exchange resins suitable for this invention are themselves wellknown. A particularly suitable basic tertiary amine ion exchange resinis sold by Rohm and Haas Company under the trademark Amberlyst A-21which has a density of about 37 to 42 lbs/ft.³, a minimum anion exchangecapacity of 4.2 meq./g. of dry resin, a surface area of about 30 to 40m² /g, and an average pore diameter of about 900 to 1300 Angstrom units.Suitable acidic resins are any sulfonated polystyrene type resin and aparticularly suitable acidic resin is sold by Rohm and Haas Companyunder the trademark Amberlyst 15 which has a bulk density ofapproximately 595 g/l., a hydrogen ion concentration of approximately4.9 milliequivalents/g. dry, a surface area of from about 40 to 50 m²/g. and an average pore diameter of from about 200 to 600 Angstromunits. The amount of ion exchange resin used is of such a magnitude thatthe weight ratio of condensation-rearrangement product treated to acidicresin should be between 0.1 and 1.0 and the weight ratio of producttreated to basic resin should be between 0.05 and 1.0.

Both the condensation-rearrangement reaction product and the ionexchange resins must be essentially dry since the presence of moisturewould result in carbamate hydrolysis catalyzed by the resins.Accordingly, a nonaqueous solvent may be used to prepare the resin bedand as solvent for the condensation rearrangement reaction producttreated. Suitable solvents are nitrobenzene, toluene, xylene, benzene,alcohols, ethers and ketones. Nitrobenzene, ethanol, dimethyl ether anddimethyl ketone are especially suitable. Nitrobenzene is preferredinasmuch as it is frequently used as the reaction solvent in thecondensation-rearrangement reaction.

The following examples are provided to illustrate the process inaccordance with the principles of this invention but are not to beconstrued as limiting the invention in any way except as indicated bythe appended claims.

EXAMPLE 1

A condensation-rearrangement product was prepared according to thefollowing procedure (see U.S. Pat. No. 4,146,727): A condensationproduct from the reaction of ethylphenylcarbamate with a 30 percentaqueous formaldehyde solution and 37 weight percent hydrochloric acid inwater was prepared according to Example 2 of U.S. Pat. No. 2,946,768 andcontained approximately 33 percent unreacted ethylphenylcarbamate, 38percent diphenylmethane dicarbamates (2,4' and 4,4'-isomers), 4 percenttriurethanes, 15 percent N-benzyl compound dimer (2 and4-[(ethoxycarbonyl) phenylaminomethyl]phenylcarbamic acid, ethyl ester),8 percent N-benzyl compound trimers such as4[(ethoxycarbonyl)phenylaminomethyl]-2,4'-methylenebis(phenylcarbamicacid)diethyl ester and a small amount of other unidentified by-products.Eighteen g. of the condensation reaction product along with 18 g. ofnitrobenzene solvent and 6.0 g. of 96.4 weight percent sulfuric acidwere charged to a reaction flask and heated at 80° C. for 30 minutes.After completion of the reaction and acid extraction, analysis of theproduct showed 100 percent conversion of the N-benzyl compound dimersand trimers to the desired methylene group bridged aromatic di- andtriurethane compounds. After removal of solvent the reaction mixturecontained 0.10 wt. % total NH₂ groups. Anhydrous Amberlyst 15 resin(22.8 g.) were slurried with nitrobenzene and charged to a one inch i.d.glass column 2.5 ft. long. Anhydrous Amberlyst A-21 (60.8 g.) wereslurried with nitrobenzene and charged to the same column. Afterbackflushing with nitrobenzene, four bed volumes of nitrobenzene werepassed through at a rate of four bed volumes per hour. At this point, asolution containing 50 wt.% condensation-rearrangement product innitrobenzene (17 g. product+18 g. nitrobenzene) was added to the columnand eluted with four bed volumes of nitrobenzene. The eluent wasdistilled to remove nitrobenzene giving 18 g. of treated product as aresidue.

The treated condensation-rearrangement product was pyrolyzed to form apolymeric isocyanate as follows. Eighteen g. of product was dissolved in1 l. of diphenyl ether containing 7 ppm aluminum acetylacetonate. Thissolution was added to a 2 l. round bottom flask equipped with a nitrogensparger, heater, and 1 ft. Vigeraux column and overhead condenser. Thesolution was heated to 250° C. and purged with nitrogen at 1 l./min.After 80 to 100 ml. of overhead were collected, analysis showed noethanol in the overhead. At this point, the flask was cooled to 130° C.and the diphenyl ether distilled off under vacuum with a final pressureof 0.5-1.0 mm Hg. The isocyanate content of the residue was determinedby titration and expressed as wt.% NCO. Untreatedcondensation-rearrangement product was pyrolyzed in an identicalprocedure. The results are summarized in Table A below.

EXAMPLE 2

The procedure of Example 1 was repeated except that 50 g. of reactionproduct were treated in a resin bed contained 50 g. of acidic resin and80 g. of basic resin. The results are summarized in Table A.

EXAMPLE 2a (Comparison)

To illustrate the importance of the joint use of the acidic and basicresins, Example 2 was repeated but the basic resin was omitted from thebed. The results in Table A show a higher isocyanate content for Example2 treated product than for Example 2a.

EXAMPLE 3

The procedure of Example 1 was repeated except ethanol was used in placeof nitrobenzene as solvent and 21.8 g. of product contacted a monobed of15.2 g. acid resin with 40.5 g. basic resin at an elution rate of 3. Theresults are summarized in Table A.

EXAMPLE 4

The procedure of Example 3 was repeated with 30.0 g. of product beingtreated. The results are summarized in Table A.

EXAMPLE 5

The procedure of Example 1 was repeated with toluene replacingnitrobenzene as solvent and 66 g. of reaction product was contacted by amonobed containing 30 g. acidic resin and 80 g. basic resin. The resultsare summarized in Table A.

EXAMPLE 5a

The procedure of Example 5 was repeated except the monobed contained 190g. acidic resin and 1310 g. basic resin. The results are summarized inTable A.

EXAMPLE 6

The procedure of Example 1 was repeated except that 30 g. of reactionproduct contacted a monobed containing 15.4 g. acidic resin and 40.5 g.basic resin. The results are summarized in Table A.

EXAMPLE 7

The procedure of Example 6 was repeated except the monobed contained 60g. acidic resin and 160 g. basic resin. The results are summarized inTable A.

                                      TABLE A                                     __________________________________________________________________________    Treatment Procedure        Elution.sup.2                                                                      wt. % NCO after                               Example                                                                            Solvent                                                                               g Prod.                                                                           g A-15.sup.1                                                                       g A-21.sup.1                                                                       Rate Pyrolysis                                     __________________________________________________________________________    1    nitrobenzene                                                                         18   22.8 60.8 4    30.8 (30.0).sup.3                             2    nitrobenzene                                                                         50   50.0 80.0 4    29.7 (28.9)                                    2a  nitrobenzene                                                                         50   50.0 0.0  4    29.3 (28.9)                                   3    ethanol                                                                              21.8 15.2 40.5 3    30.5 (30.0)                                   4    ethanol                                                                              30.0 15.2 40.5 3    29.2 (27.6)                                   5    toluene                                                                              66   30.0 80.0 4    30.9 (30.1)                                    5a  toluene                                                                              66   190.0                                                                              1310.0                                                                             4    31.5 (30.1)                                   6    nitrobenzene                                                                         30   15.4 40.5 4    30.7 (28.0)                                   7    nitrobenzene                                                                         30   60.0 160.0                                                                              4    31.5 (28.0)                                   __________________________________________________________________________     .sup.1 Anhydrous wts. of Amberlyst15 (A15) and Amberlyst A21 (A21)            .sup.2 Bed volumes/hour                                                       .sup.3 Value in () obtained on untreated material.                       

What is claimed is:
 1. In the process of producing polyisocyanates by(a)condensing an alkyl-N-phenylcarbamate having 1 to 3 carbons in the alkylmoiety in the presence of an acid to produce condensate containingdiphenylmethane dicarbamates and polymethylene polyphenyl carbamateswith by-product N-benzyl compounds, rearranging said N-benzyl compoundsin said condensate with acid catalyst to obtain a pyrolysis feed mixturecontaining diphenylmethane dicarbamates, polymethylene polyphenylcarbamates and 0.02 to 1.0 percent by weight of amine impurities as NH₂groups, and (b) thermally decomposing the carbamate moieties in thepyrolysis feed mixture to isocyanate moieties to producepolyisocyanates, the improvement comprises increasing the percentisocyanate content of said polyisocyanates by prior to step (b)converting any amine salt by-products in said pyrolysis feed mixture tofree amine by-products by contacting the mixture with a weakly basictertiary amine anion exchange resin and removing any free amineby-products by contacting the feed mixture with a strongly acidicsulfonated polyaromatic ion exchange resin whereby the amine impurity inthe pyrolysis feed mixture is reduced to less than 0.001 percent byweight as NH₂ groups.
 2. The process of claim 1 wherein a solventsolution of said pyrolysis feed mixture containing said amine and aminesalt by-products is passed through a mixed bed of said weakly basictertiary amine anion exchange resin and said strongly acidic sulfonatedpolyaromatic ion exchange resin.
 3. The process of claim 2 wherein saidsolvent is selected from the group consisting of nitrobenzene, toluene,xylene, ethanol, dimethyl ether and dimethyl ketone.